cifs: fix race between call_async() and reconnect()

cifs_call_async() queues the MID to the pending list and calls
smb_send_rqst().  If smb_send_rqst() performs a partial send, it sets
the tcpStatus to CifsNeedReconnect and returns an error code to
cifs_call_async().  In this case, cifs_call_async() removes the MID
from the list and returns to the caller.

However, cifs_call_async() releases the server mutex _before_ removing
the MID.  This means that a cifs_reconnect() can race with this function
and manage to remove the MID from the list and delete the entry before
cifs_call_async() calls cifs_delete_mid().  This leads to various
crashes due to the use after free in cifs_delete_mid().

Task1				Task2

cifs_call_async():
 - rc = -EAGAIN
 - mutex_unlock(srv_mutex)

				cifs_reconnect():
				 - mutex_lock(srv_mutex)
				 - mutex_unlock(srv_mutex)
				 - list_delete(mid)
				 - mid->callback()
				 	cifs_writev_callback():
				 		- mutex_lock(srv_mutex)
						- delete(mid)
				 		- mutex_unlock(srv_mutex)

 - cifs_delete_mid(mid) <---- use after free

Fix this by removing the MID in cifs_call_async() before releasing the
srv_mutex.  Also hold the srv_mutex in cifs_reconnect() until the MIDs
are moved out of the pending list.

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <sfrench@localhost.localdomain>

1 files changed, 4 insertions, 2 deletions

diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index 2a24c524fb9a..87abe8ed074c 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -576,14 +576,16 @@ cifs_call_async(struct TCP_Server_Info *server, struct smb_rqst *rqst,
 	cifs_in_send_dec(server);
 	cifs_save_when_sent(mid);
 
-	if (rc < 0)
+	if (rc < 0) {
 		server->sequence_number -= 2;
+		cifs_delete_mid(mid);
+	}
+
 	mutex_unlock(&server->srv_mutex);
 
 	if (rc == 0)
 		return 0;
 
-	cifs_delete_mid(mid);
 	add_credits_and_wake_if(server, credits, optype);
 	return rc;
 }